Using sequential images to understand ocean current speed

Time-lapse images used in a series can help to give a clear impression of motion. However, using this method in order to learn more about ocean currents or measure their speed can be challenging.

To resolve this, PML scientists have applied an old technique, with a new twist, utilising the latest satellite data to enhance our knowledge and improve accuracy in measuring current speed.

In a new study led by PML scientists, a 30-year old method, known as ‘Maximum Cross Correlation’, was combined with new high-resolution optical data from a geostationary satellite in orbit above the Korean Peninsula. The study, which used analysis techniques from medical imaging, demonstrates that a total current can be measured at high-resolution using images only a few hours apart The accuracy of ~0.2 m/s is sufficient to visualise eddies and to monitor tidal flow in the Yellow Sea, where tidal currents of up to 0.8 m/s transportsediment from the Yangtze river.

Lead author PML’s Mark Warren commented: “in good conditions, this method could supplement traditional techniques to give a clearer picture of ocean currents. Through PML's involvement in the GlobCurrent project, many users such as shipping companies, search and rescue and environmental agencies could benefit in the future from more detailed information on ocean currents.”

This study is part of the ongoing collaborative project GlobCurrent (funded by the European Space Agency), whose aim is to extract current information from a wide variety of spaceborne sensors and combine them in an optimal way to improve our mapping of ocean currents. Understanding these currents is essential for many reasons, including monitoring the distribution of pollution and debris. Also, climate scientists need to understand ocean currents because they transport heat and therefore play a decisive role in influencing the climate of regions through which they flow. Ocean currents can also be used for marine power generation.

Image above right: An eddy (rotational feature) is identified in the currents to the northeast of Tsushima Island, with the colours showing the sea surface temperature. Right-hand figure shows the velocities across the dotted section. These have been derived from the method outlined in the study.

A new international report “An updated synthesis of the impacts of ocean acidification on marine biodiversity” shows beyond doubt that ocean acidification is an issue of serious environmental and policy concern.